High-level phylogenetic analysis using developmental sequences: The Cambrian †Martinssonia elongata, †Musacaris gerdgeyeri gen. et sp. nov. and their position in early crustacean evolution

The ontogenetic sequence of †Martinssonia elongata, a derivative of the stem lineage of Eucrustacea, has been re-investigated. Morphological and morphometric data provide a revision of the original description of this species. Specimens originally assigned to second and third developmental stages have been removed from the †M. elongata sequence and assigned, together with several larger specimens, to an entirely new species, †Musacaris gerdgeyeri gen et. sp. nov having a completely unsegmented body, giving the appearance of a giant euarthropod head larva. This is interpreted either as a hypertrophied larva or a late developmental stage of a neotenic species. Only the earliest unsegmented larvae and segmented instars of the original sequence are ascribed to †M. elongata. The two species are apparently closely related and are closer to Labrophora (†Phosphatocopina + Eucrustacea) than to other Cambrian “‘Orsten’ crustacean stem derivatives”. The ontogenetic sequences of the two taxa and those of the other derivatives of the recently investigated labrophoran stem lineage indicate several heterochronic peramorphic (‘adultising’) events during early crustacean evolution. This is most evident in the development of the proximal parts of the appendages.